$session = $this->facebook->getSession();
$logoutUrl = $this->facebook->getLogoutUrl(array('next' => base_url(), 'session_key' => $session['session_key']));
setcookie('fbs_'.$this->facebook->getAppId(), '', time()-100, '/', '.domain.com');
$this->session->sess_destroy();
redirect($logoutUrl);
I got it from this link http://forum.developers.facebook.net/viewtopic.php?id=71219.
Hope it help someone.